The present invention relates to a pressure measurement device and, in particular, to a pressure measurement device for accurately measuring pore-air pressure produced by the detonation of explosives.
The Defense Nuclear Agency (DNA) has sponsored an ongoing effort to evaluate the survivability of structures under loading conditions produced by nuclear detonations. To this end, high-explosive charges are typically used to simulate the nuclear loading phenomenon on test structures. An important aspect of the simulation is the ground motion produced by the charge detonation. In order to properly position test structures which are sensitive to ground motion, it is necessary to accurately predict the ground motion field produced by the explosive charge.
Attempts to calculate the ground motion produced by large-scale explosive charges have been somewhat successful at duplicating the early-time motions (i.e., those occurring during the positive air blast loading phase). However, these calculations have not accurately modeled late-time motions. The material properties thought to contribute most to late-time behavior are shock degradation of the cohesive strength of cemented soils, and the ability of air trapped within the soil, known as pore-air, to transmit pressure when compressed. Pore-air effects are of greater importance near the ground surface, where air blast pressures penetrate into the soil. Information on the soil pore-air pressure history is essential for improvement of calculated models for late-time ground motion near the ground surface.
Pore-air pressure measurements are complicated by the presence of high shock transients, and the fact that pore-air pressure changes during shock loading are relatively small. No method is currently available to accurately and consistently measure pore-air pressure.
The key to obtaining an accurate and reliable measurement of the pore-air pressure in soil is to isolate the pressure sensing element from the stresses transmitted through the soil particles while allowing the pore-air pressure to flow to the sensor. This can be accomplished by placing an air permeable filter in front of the pressure sensor. This filter must have sufficiently small pore size as to effectively block out the finest soil particles while allowing air to flow through at the highest possible rate. In addition, the filter must be corrosion resistant and strong enough not to fail under significant loads.